The present invention relates to glass articles comprising multiple layers of glass, and to a method for manufacturing such articles. The method is useful for processing viscous glass melts, and in preferred embodiments, permits the continuous forming of a multilayer or graded-index glass product from multiple feed streams of molten glass.
Glass manufacturing processes wherein multiple glass streams are supplied to forming apparatus for the manufacture of layered glass are well known. For example, U.S. Pat. No. 2,052,269 describes apparatus for forming thermometer tubing comprising multiple glass layers or components wherein a primary molten glass and a secondary contrasting molten glass are simultaneously fed to a drawing orifice configured so that a stripe of the contrasting glass is embedded in the primary glass as glass capillary tubing for thermometers or the like is drawn. Nozzles for the two glasses within the drawing orifice may be shaped to control the cross sections of the contrasting and primary tubing glass to form finished tubing suitable for various instrumentation applications. U.S. Pat. No. 3,573,024 describes modified apparatus suitable for drawing similar capillary tubing but being designed to avoid the introduction of seeds or air bubbles in the contrasting glass for the capillary stripe.
Diverse streams of molten glass have also been combined for the purpose of producing colored glass products. However, in this case, the positioning of the stream of secondary colorant glass within the primary glass stream is relatively unimportant, the objective being to achieve thorough mixing of the colorant glass in the primary glass in order to provide a homogeneously colored glass product. U.S. Pat. No. 3,445,216 describes apparatus and methods for providing colored glasses using a secondary stream of highly colored glass produced by melting a glass color frit. The combined glass streams traverse multiple stirring stations wherein thorough mixing of the streams to obtain color homogeneity in the final mixed glass feed stream is achieved. The stirring or mixing stations utilize rotating mixers of paddle or screw design to obtain shear mixing of the glass flowing around and past the stirrers.
Stirring apparatus for insuring homogeneity in glass melting operations producing large volumes of glass at high melting rates has also been highly developed. U.S. Pat. No. 3,725,025, for example, describes stirring apparatus wherein a glass feed stream is subdivided into multiple small feed streams which are mixed by shearing between the faces of opposed, relatively rotatable disc elements. U.S. Pat. No. 3,819,350 describes apparatus for melting glass at a high rate wherein homogenization of the glass is accomplished by shear mixing in an annular mixing region formed by a rotating cylinder and the cylindrical side walls of the mixing chamber. In both cases, the shearing of the molten glass is continued for a time sufficient to obtain thorough mixing and homogenization of the glass supplied to the mixing zone, so that glass exiting the mixing zone will not exhibit cords or other glassy inclusions which would impart non-uniform refractive index or other chemical or physical properties in glass articles produced therefrom.
Glass products exhibiting nonuniform physical properties such as refractive index gradients or the like are becoming increasingly important in fields of optical technology. Of particular commercial importance has been the development of glass optical fibers useful for transmitting information optically at high rates over long distances following the path of the optical fiber. Present-day glass optical fibers are drawn from glass preforms in which radial refractive index gradients and/or other, more complex radial refractive index distributions have been generated.
A common technique for manufacturing glass preforms for optical fibers or the like involves forming multiple layers of glass on a starting member or bait, generating the desired refractive index profile as the various layers of glass are applied. U.S. Pat. No. 3,823,995 describes one such method for making a multilayer glass article wherein glass layers of desired refractive index are sequentially deposited on the glass preform as fine particles of glass of appropriate composition.
Glass layering processes such as utilized for these and other optical applications are most frequently accomplished in a batch mode, i.e., by the fabrication of discrete preforms which must then be further processed in multiple stages to provide the finished product. However, alternative methods for continuously making graded-index optical devices from molten glass have been developed, as shown for example in U.S. Pat. No. 3,614,197. In that patent, multiple glass melts are prepared and simultaneously supplied to a forming orifice comprising concentric annular outlets for forming multiple-layer glass rod or fiber.
The latter approach to the manufacture of multilayer glass products requires the melting of multiple glass melts of differing composition, and becomes increasingly complex as the number of glasses required for article manufacture increases. The method of U.S. Pat. No. 3,823,995, supra, permits the generation of more complex refractive index profiles, but is generally limited to non-continuous manufacture.
U.S. Pats. Nos. 3,507,940, 3,507,946 and 3,551,955 describe a mixing prodedure for producing articles incorporating a two- or three-dimensional dielectric constant gradient wherein adjacent shaped charges of particulate material are stirred to achieve controlled intermingling of the charges. Again, however, the method is limited to non-continuous manufacture and the stirring procedures employed are likely to generate turbulent and/or secondary flows and loss of fine structure in viscous liquid materials such as glasses.
It is therefore a principal object of the present invention to provide an improved method for the fabrication of multilayer glass articles.
It is a further object of the invention to provide a method for the manufacture of multilayer or graded-refractive-index glass articles using two or a relatively small number of different glass compositions.
It is a further object of the invention to provide glass products such as glass rod, tubing, or fiber exhibiting a predetermined non-uniform radial composition profile.
Other objects and advantages of the invention will become apparent from the following description thereof.